Perpetual floating drops

TL;DR

This paper analyzes the stability and lifetime of floating drops on liquid surfaces, linking it to van-der-Waals forces and surface motion, with implications for controlling liquid surfaces using floating or rotating drops.

Contribution

It introduces a model connecting drop stability to van-der-Waals forces and surface motion, predicting conditions for infinite lifetime of floating drops.

Findings

Rotating or moving drops can have infinite lifetime above a critical velocity.

The minimal air film thickness is related to the Hamaker constant.

Long-living rotating paraffin droplet observed on a candle.

Abstract

The stability of floating drops on the liquid surface of the same liquid is considered in terms of viscous drainage theory. We have expressed the minimal thickness of the air film, separating the drop from the liquid surface, and the lifetime of the drop through the Hamaker constant, characterizing the intensity of van-der-Waals forces which make the air film unstable. It is shown that a horizontally moving or just rotating drop can have an infinite lifetime when the drop surface velocity exceeds some critical value. A spectacular example of a long-living rotating paraffin droplet on a burning candle is given. Results obtained can help to control a liquid surface in situ with the use of floating/rotating drops.